ChemMatCARS Nuggets - Materials Science

Atomic-Scale Surface Demixing in a BiSn Liquid Alloy

Surface properties of most materials are strikingly different from those of the bulk. This is of great consequence for nanotechnology, since function of nanoscale devices is primarily defined by new interfacial physics rather than known bulk behavior. We use resonant x-ray scattering techniques at the ChemMatCARS synchrotron facility at the Advanced Photon Source to probe surface structure of low-melting point eutectic BiSn alloy, a technologically important material in a recent drive to eliminate environmentally harmful lead-based solders. Previous studies of alloys reveal surface segregation limited only to top surface layer.

Our study shows that segregation effects can extend well below the surface monolayer: Bi-rich surface layer is followed by Sn-rich second layer and Bi-rich third layer. Our discovery of atomic-scale demixing indicates strong enhancement of inter-species interactions between Bi and Sn atoms at the surface, while interactions in the bulk are relatively weak.

X-ray resonant reflectivity (Fresnel-normalized) data from Bi43Sn57 liquid surface in the vicinity of Bi adsorption edge. Lines represent 3-layer model. Inset demonstrates 3-layer model fit, compared to those of 2-layer and 1-layer models

Resulting density composition profiles: the surface monolayer contains 96% Bi (Bi-rich), the 2nd layer 25% Bi (Sn-rich) and the 3rd layer 53% Bi (slightly Bi-rich).

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